1. Field of the Invention
The present invention relates to an air outlet mode control in a vehicular air-conditioning system, especially a control that improves the cooling sensation felt by passengers when the system automatically controls the temperature of air blown into a passenger compartment toward a target air temperature.
2. Description of the Related Art
A vehicle air-conditioning system conventionally controls a blown air temperature toward a target air temperature. Japanese Examined Patent No. Hei 6-59774 discloses a vehicle air-conditioning system. This system has the above-described automatic control feature which improves a passenger's cooling sensation by setting the air outlet mode to a bi-level mode when the cooling load is high. This blows cool air from both a face opening and a foot opening, to an upper body region and a foot region of a passenger, respectively.
In the above-described related technology, however, the cooling load evaluation is based on the target air temperature. If the target air temperature is equal to or higher than a certain temperature, namely 0 deg. C. or higher, the face mode will automatically be selected. This will blow cold air from the face opening to the upper body region of the passenger in the passenger compartment. Only if the target air temperature drops below 0 deg. C., the bi-level mode will automatically be selected. In practice, this control method causes the following problem.
The target air temperature is required to maintain the passenger compartment at a temperature set by the passenger, regardless of thermal load fluctuations in the vehicle. To achieve this result, the target air temperature is calculated based on the set temperature, the passenger compartment temperature (inside air temperature), an outside air temperature value, and a sunlight amount value.
This gives rise to a wide variation in the actual temperature of the air blown from the heat exchanger (evaporator) for cooling, which is affected by cooling load variations, even when the target air temperature is in an extremely low sub-zero temperature area (see FIG. 4 described later). In other words, even under the same condition of sub-zero target air temperature, the temperature of the air blown from the evaporator is high when the cooling load is high—such as during the time just after starting the air-conditioning system—while the air temperature blown from the evaporator drops when the cooling load drops from prolonged operation after starting the air-conditioning system.
Suppose a case where the bi-level mode is selected simply because the target air temperature meets the sub-zero condition. In this case, even when the blown air temperature is lowered due to a decrease in cooling load, the bi-level mode is maintained, and excessively cools his or her foot region by continuing to blow cold air from the evaporator. Accordingly, the passenger's sensation of cooling (amenity) may actually deteriorate.